This invention relates to a method for making glycols, preferably ethylene glycol, from alkylene oxide and water.
Alkylene glycols, such as ethylene glycol and propylene glycol, are widely used as raw materials in the production of polyesters, polyethers, antifreeze, solution surfactants, and as solvents and base materials in the production of polyethylene terephthalates (e.g. for fibers or bottles). Commercial processes for the preparation of alkylene glycols typically involve the liquid phase hydration of the corresponding epoxide in the presence of a large molar excess of water (see, e.g., Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 11, Third Edition, page 929 (1980)). When epoxides react with water to form monoglycols or with the hydroxyls on the monoglycols to form diglycols, a large amount of energy is released (about 20 kcal/mole of epoxide). If not removed from the reaction system, this energy causes the reaction medium's temperature to increase significantly. In some processes, it is imperative that the reaction energy be removed, while in others it is desired to allow the reactants to absorb the energy and heat up.
Typically, the reaction is carried out in two different types of commercial reactor practices. In one method, adiabatic operation, no heat is removed from the reactor. The temperature rise is controlled by feeding a large excess of water to allow the heat to be absorbed by the water feed. The adiabatic reactor is usually a cylindrical vessel or series of vessels with no heat transfer between vessels, operated in plug flow manner to obtain maximum monoglycol selectivity. In a second method, nonadiabatic operation, heat is removed from the reactor by transferring it to a coolant as the reaction proceeds. Here the combined feed of water and epoxide is fed to a heat exchange reactor and the heat is immediately removed by the heat exchanger as it forms. With appropriate controls and reactor design, nearly isothermal conditions can be maintained and the reaction product leaves at about the same temperature as the feed because the heat of reaction is removed by the coolant. This type of reactor is most often a shell-and-tube heat exchanger used as a reactor (referred to as a tubular, multitubular, isothermal, or heat exchange reactor), where the reaction mixture passes through several long narrow tubes, and a coolant passes on the outside of the tubes. This type of reactor is generally operated in plug flow manner to obtain maximum monoglycol selectivity.
The primary byproducts of hydrolysis reactions are di-, tri-, and higher glycols. However, as compared to monoalkylene glycols, the demand for di-, tri-, tetra-, and polyalkylene glycols is low. The formation of the polyglycols is due to the reaction of the epoxide with alkylene glycols. As epoxides are generally more reactive with glycols than they are with water, the aforementioned two commercial reactor types generally require an even greater excess of water in order to favor a commercially attractive selectivity to the monoglycol product. For example, a typical commercially practiced method for making ethylene glycol has a molar selectivity to monoethylene glycol (MEG) of about 88% at a water to ethylene oxide (EO) mass feed ratio of 8:1, about 20 times the stoichiometric amount of water required for complete reaction. Selectivity is calculated by dividing the number of moles of EO consumed to form a given product divided by the total number of moles of EO converted to all products. However, even in light of such large excesses of water, it would be desirable for the selectivity to the monoalkylene glycols to be even higher. In addition, increasing the water to epoxide feed ratio also increases the cost of distilling water from the glycol. Thus, there is much interest in alternative processes that increase monoalkylene glycol selectivity without increasing production costs.
A number of publications show that higher selectivity to monoalkylene glycols can be achieved if the reactions are conducted using heterogeneous catalytic processes, such as with anion exchange resin catalysts. See, for example: EP-A-156,449 (metalate-containing anion exchange resins); JP-A-57-139026 (anion-exchange resin in the halogen form); Russian Patent Nos. 2002726 and 2001901 (anion exchange resin in the bicarbonate form); WO/20559A (anion exchange resin); WO 97/33850 (anion exchange resin); and co-pending U.S. Provisional Patent Application No. 60/069,972, filed Dec. 18, 1997. U.S. Pat. Nos. 4,701,571 and 4,982,021 disclose various possible reactor configurations for the production of alkylene glycols using metalate anion exchange resin catalyst, and these references also suggest that isothermal reactors may be preferred because the amount of catalyst required may be less than that required in other types of reactor systems. Russian Patent No. 2001901 also discloses a method for catalytic production of alkylene glycols in a plug flow reactor or in a series of reactors with epoxide feed distributed to each reactor, where the reactors are 55-100% filled with bicarbonate-containing anion exchange resin catalyst, and the reactors are isothermal reactors.
Interestingly, none of the catalyst systems disclosed in the publications mentioned above have been successfully implemented for commercial production of alkylene glycols. The metalate-based resin system is not commercially attractive because metalate ions leach from the anion resin and contaminate the glycol product.
Moreover, anion exchange resin catalysts are believed to be compromised by limited tolerance to heat. As described in WO97/19043, monoglycol selectivity deterioration at high temperatures is the only reason why these catalysts have failed to achieve commercially viable prominence. Neither this reference or any of the aforementioned prior art mentions any problems associated with swelling of the catalyst resin.
It is known that the process of exchanging ions cause anion exchange resins to swell, as do the presence of solvents. See, for example: Ion Exchangers, K. Dorfner, Ed., (Walter de Gruyter: Berlin), 1991; and C. E. Harland, Ion Exchange: Theory and Practice, 2nd ed., (Royal Society of Chemistry: Cambridge), 1994. This type of swelling is reversible, and the extent of swelling is limited to no more than about 20% volume increase for ion exchange and about 100% for solvent swelling (based on a wet resin that is pre-swollen with water from its dry form), with no additional swelling even with prolonged exposure to the ions or solvent. The extent of this type of swelling depends on the type and concentration of the ions or solvent and on the type of resin matrix and extent of crosslinking. However, resin swelling under epoxide hydrolysis reaction conditions is different than swelling caused by solvents or ion exchange processes, yet, none of the above mentioned references teaches or suggests a mechanism for minimizing resin swelling during alkylene glycol production.
It is desirable to have a process for making alkylene glycols commercially, which minimizes and controls resin catalyst swelling and permits optimal temperature control for effective use of temperature-sensitive heterogeneous catalyst materials such as anion exchange resins.